Length or angle measuring apparatus with differential screw for moire adjustment

ABSTRACT

The invention relates to a length or angle measuring apparatus, wherein a holding device ( 6 ) that can be turned by means of a differential screw ( 16 ) and a flector joint ( 5 ) is used in order to perform an adjustment with the greatest possible sensitivity of the angle between a scale ( 2 ) and a scanning disc ( 9 ). Advantages in terms of vibration resistance of the array and the amount of components required are obtained by combining the differential screw ( 16 ) with a flector joint ( 5 ).

[0001] The present invention is directed to a length- or angular measuring instrument according to the definition of the species in claim 1. Measuring instruments of this kind are primarily used to determine the relative displacement or absolute position of movable machine parts, for example in machine tools or measuring machines.

[0002] In length and angular measuring instruments of the mentioned type, a material measure, e.g., in the form of an optical grating, is scanned by a sensor which is mounted behind a scanning plate constituted as another grating. An exact adjustment of the angle between the two gratings is necessary, in this context, to avoid the moiré effect produced when working with gratings which are tilted towards one another, in that the two gratings are positioned in parallel, or to intentionally produce a moiré effect in that a specific angle is precisely adjusted. When a plurality of tracks is applied to the material measure, the effect of angular errors which are too great is that the relationship among the tracks is lost. In some situations, a reference mark is no longer able to be allocated to the correct period of an incremental graduation.

[0003] To adjust the moiré angle between the material measure of a scale and the scanning plate of a scanning unit, the German 2844066 proposes a lever arm which is secured, on one side via a flector joint to a movable or stationary machine part and, on the other side, has an adjustment screw, which is supported on this machine part and which bears the scanning head, together with the scanning plate. Thus, when the screw is adjusted, the lever arm, together with the scanning unit, swivel, so that the moiré angle is adjusted, in turn.

[0004] The disadvantage of this system is that it does not permit a precise adjustment of the moiré angle. Given a typical lever arm length of 50 mm and an adjustment screw having a screw pitch of 0.8 mm, an angular change of 16 mrad results for one turn of the screw. Moreover, in this system, the lever arm is only protected in one direction against vibrations. For that reason, the scanning unit must be secured by an additional clamping screw.

[0005] It is known, as a fine-adjustment gear, to use a dual screw gear having a differential screw, as described, for instance, by S. Hildebrand: Feinmechanische Bauelemente, Hanser Publishers, Munich, Vienna 1978, 3rd edition, pp. 534-536. A dual screw gear permits a precise adjustment in the linear direction, since, in the context of one screw turn, the change in the distance between two objects joined by the differential screw corresponds to exactly the difference between the two pitches of the differential screw.

[0006] The object of the present invention is, therefore, to devise a length or angular measuring instrument which permits a fine adjustment of the moiré angle, a low-vibration attachment of the scanning unit, and thereby makes do with few components.

[0007] This objective is achieved by a device having the features set forth in claim 1. Advantageous specific embodiments are derived from the measures delineated in the dependent claims.

[0008] A lever, which is secured via a flector joint to the fixed or movable machine part, is now able to be adjusted in that, instead of a customary screw resting on this machine part, a differential screw is provided, which is screw-coupled by its first thread to the lever and, by its second thread, to this machine part. If, at this point, one turns the screw by one rotation, the distance of this lever end to this machine part changes precisely by the difference in the pitches, when the directional sense of the pitch is the same. As a result, the lever changes its inclination, since it is executing a rotary motion about the flector joint. In the above mentioned example, given thread pitches of 0.7 mm and 0.8 mm, a change in distance of 0.1 mm results for one rotation of the differential screw, and thus an angular change of approximately 2 mrad. Thus, the adjustment may be carried out in a clearly more precise manner than previously possible. Since the dual screw coupling now provides protection from vibrations in two directions, the need is eliminated for an additional clamping of the lever. In addition, the differential screw is pretensionsed by the axial forces which arise in response to deformation of the flector joint. Moreover, the differential screw is clamped by the mutual tilting of its two drilled holes; it is thereby screw-coupled in a self-locking manner.

[0009] Further advantages of, as well as details pertaining to the present invention are revealed in the following description of FIG. 1, which shows a design in accordance with the present invention of a device for adjusting a scanning head.

[0010] In FIG. 1, one can discern a scale 1 having a material measure 2 in the form of an incremental graduation. In the example, scale 1 is permanently connected to the machine (not shown). An adjustment device 3 is connected to a movable part (likewise not shown) of the machine along measuring direction X.

[0011] Adjustment device 3 is fabricated from one piece and is composed of a base 4, on which a lever in the form of a mounting support 6 is hinge-mounted via a flector joint 5. In this context, flector joint 5, designed as a web between base 4 and mounting support 6, permits a rotation of mounting support 6 about an axis perpendicularly to the plane of material measure 2.

[0012] Mounting support 6 has an opening 7 for receiving scanning unit 8. Scanning unit 8 includes a scanning plate 9, whose alignment must be adjusted with respect to material measure 2 in order, for example, to avoid moiré effects or to achieve such effects to the desired degree. A sensor (not drawn) is situated behind scanning plate 9, or the sensor itself is structurally designed to fulfill the function of scanning plate 9.

[0013] On the side opposite flector joint 5, mounting support 6 has a bore 10 having internal screw thread 11. Also discernible on base 4 is an arm 12, which bears a bore 13 having internal thread 14, so that bore 10 and bore 13 are situated on a common axis. Between mounting support 6 and base 4, a gap 15 remains which allows mounting support 6 to tilt about flector joint 5. The diameter of bore 10 is larger than the diameter of bore 13. This enables a simple pushing-through and bolting-on action when installing differential screw 16. However, given the same diameters, other design approaches are conceivable.

[0014] The pitch of internal thread 11 is larger than the pitch of internal thread 14. In one half, a differential screw 16 has a thread 11.1, whose pitch and diameter match internal thread 11; in the other half, a thread 14.1, whose pitch and diameter match internal thread 14.

[0015] Prior to mounting scanning unit 8, differential screw 16 is screw-coupled to base 4 and mounting support 6, in order to pretension flector joint 5. Because of the different pitches of internal threads 11 and 14, gap 15 is enlarged or reduced in the process, depending on the directional sense of internal thread 11 and 14 and the direction of rotation of differential screw 16. In this state, scanning unit 8 may then be mounted on mounting support 6 and, following a rough adjustment, be bolted on at bores 17 and 18. By turning differential screw 16, the angle between scanning plate 9 and material measure 2 may now be adjusted in that mounting support 6 is rotated around flector joint 5.

[0016] The width of gap 15 between bores 10 and 13 changes in response to a turn of differential screw 16 precisely by the difference in the pitches of internal threads 11 and 14; thus, the adjustment may be carried out with exceptional precision.

[0017] Since the two-sided screw coupling provided by differential screw 16 prevents mounting support 6 from moving in the direction of bores 10 and 13, no more vibrations occur in this direction. Flector joint 5 is blocked; the need is eliminated for an additional clamping screw.

[0018] If, moreover, as shown, flector joint 5 is positioned to lie with the common center of mass SP of scanning unit 8 and mounting support 6 on an axis in parallel with measuring direction X, then in response to acceleration of adjustment device 3 in measuring direction X, no moment of torque occurs with respect to the above defined axis of rotation of flector joint 5. This further enhances the stability of the system.

[0019] The adjustment device is further strengthened when arms 12 and 19 of base 4 are reinforced by ribs 20 and 21 which run in measuring direction X and perpendicularly to scale 1.

[0020] In a variation of the present invention, the adjustment device according to the present invention may be integrated in the scanning unit and, in that way, only the scanning plate itself or a sensor acting as a scanning plate is adjusted.

[0021] The described principle for adjusting scanning units is advantageously not limited to applications in length and angular measuring instruments based on optical scanning. Its application in other systems is also conceivable, such as those which are based on magnetic material measures and which are scanned using magnetoresistive sensors or Hall sensors; likewise conceivable are capacitive or inductive sensors having corresponding material measures. 

What is claimed is:
 1. A length- or angular measuring instrument having a material measure (2), which is situated opposite a scanning unit (8), having an adjustment device (3), made up of a base (4) and a mounting support (6), which is joined via a flector joint (5) to the base (4) and which supports the scanning unit (8), wherein this adjustment device (3) includes a differential screw (16) having two threads (11.1, 14.1) of different pitches, which is screw-coupled by its first thread (11.1) to the mounting support (6) and by its second thread (14.1) to the base (4).
 2. The length or angular measuring instrument as recited in claim 1, wherein the base (4), the flector joint (5), and the mounting support (6) are fabricated from one piece.
 3. The length or angular measuring instrument as recited in claim 1 or 2, wherein the flector joint (5) and the common center of mass SP of the scanning unit (8) and the mounting support (6) lie on one axis in parallel to the measuring direction X.
 4. The length or angular measuring instrument as recited in one of the preceding claims, wherein the flector joint (5) and the bores (10, 13) are reinforced by ribs (20, 21) to accommodate the arms (12, 19) bearing the differential screw (16).
 5. The length or angular measuring instrument as recited in one of the preceding claims, wherein the threads (11.1, 14.1) of the differential screw (16) have different diameters.
 6. The length or angular measuring instrument as recited in one of the preceding claims, wherein the differential screw (16) is clamped in a self-locking manner by the mutual tilting of the bores (10, 13) about the flector joint (5). 